Method of preparation of alloys of refractory metals

ABSTRACT

THE PRESENT INVENTION RELATES TO A METHOD OF PREPARATION OF ALLOYS F REFRACTORY METALS, IN WHICH A SOLID HOMOGENEOUS SOLUTION OF NIOBIUM AND AT LEAST ONE OF TWO REFRACTORY ETALS SUCH AS TUNGSTEN OR MOLYBDENUM IS PREPARED UNDER AN INERT ATMOSPHERE IN A LIQUID BATH OF A CARRIER METAL (NICKEL OR COPPER), THE SAID SOLID SOLUTION IS SEPARATED FROM THE LIQUID PHASE AND THE WHOLE IS RAPIDLY COOLED SO AS TO SOLIDIFY THE LIQUID PHASE THUS SEPARTED FROM THE SOLID PHASE. THE SEPARATION OPERATION IS CARRIED OUT EITHER BY CENTRIFUGING OR BY ELECTRO-MAGNETIC MEANS AND SAID LATTER MEANS MAY ALSO CONSTITUTE THE HEATING MEANS. THESE ALLOYS ARE ESPECIALLY APPLIED TO THE MANUFACTUE OF AERONAUTICAL PARTS OR OF SUPERCONDUCTOR DEVICES.

United States Patent Omce 3,700,428 Patented Oct. 24, 1972 US. Cl. 75-10 R Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a method of preparation of alloys of refractory metals, in which a solid homogeneous solution of niobium and at least one of two refractory metals such as tungsten or molybdenum is prepared under an inert atmosphere in a liquid bath of a carrier metal (nickel or copper), the said solid solution is separated from the liquid phase and the whole is rapidly cooled so as to solidify the liquid phase thus separated from the solid phase. The separation operation is carried out either by centrifuging or by electro-magnetic means and said latter means may also constitute the heating means. These alloys are especially applied to the manufacture of aeronautical parts or of superconductor devices.

The present invention relates to a method of preparation of alloys of refractory metals, such as for example niobium-tungsten and/ or molybdenum.

At the present time, the alloys are prepared by direct co-fusion from constituents such as niobium and tungsten or molybdenum. The melting temperatures of the elements being very high (2,468 C. for niobium, 3,380 C. for tungsten, 2,615 C. for molybdenum, for example) recourse is had in practice to the use of an electric arc furnace or an electronic bombardment furnace. In the case of melting by electronic bombardment, the operation is effected by progressive enrichment with added metal, which necessitates a number of melts, generally six to eight. This method is therefore long, uneconomic and very expensive. In both cases, the melts do not make it possible to obtain alloys having a homogeneous strusture.

Alloys of two refractory metals have also been prepared by the simultaneous deposit on a support of the vapours of these refractory metals. This method is however extremely costly and is in practice only applied for the production of thin layers.

The present invention has for its starting point the discovery that certain ternary systems such as Nb-W-Cu and Nb-Mo-Cu possess at high temperature, for example of the order of l,900 C., a field having two phases, constituted by a homogeneous solid solution of Nb-W or Nb-Mo respectively in equilibrium with a liquid Cu-Nb, which field furthermore covers almost completely the range of concentrations.

The method according to the invention is essentially characterized in that all the constituents of the alloy with a non-refractory carrier metal, chosen for its high solubility in the liquid state with one of the refractory metals and low solubility with the desired alloy, in that an inert atmosphere is created around these various constituents, and in that their temperature is increased at least up to the melting point of the said vector metal but below the melting points of the most meltable refractory metal and of the alloy formed by the carrier metal and the other refractory metals, in that the temperature is maintained for a period of time suflicient to reach an equilibrium between a solid phase constituted by a solid solution of the refractory metals and a liquid solution composed of an alloy of one or more refractory metals with the said carrier metal, in that the said solid phase is separated from the liquid phase, and in that a final cooling is effected so as to solidify the liquid phase thus separated from the solid phase.

This method permits the preparation of an extremely wide range of solid solutions with a base of niobiumtungsten and/or molybdenum.

The choice of the carrier metal is determined by various considerations:

It must not have, at the temperature of preparation, high inter-metal afiinities, either for niobium, or for tungstein, or for molybdenum;

Its solubility must be very low in the solid solution of refractory metals, which is to be prepared;

On the other hand, its solubility in the liquid state with at least one of the two refractory metals must be substantial, so as to obtain an alloy (vector metal-refractory metal) which melts, over a fairly wide field of concentrations, below the melting temperature of the refractory metal;

It should preferably form with the reefractory metal a liquid alloy having a density different from that of the solid solution in equilibrium, in order that mechanical separation may be possible, or if it does not fulfil this condition, it must have differences of behviour with respect to the solid solution, under the action of an alternating electric field, which are sufiicient to carry out an electro-magnetic separation.

It has been found that, by way of example, copper and nickel are metals which satisfy all these conditions.

The choice of the preparation temperature is located at the same time well above the melting temperature of the carrier metal (copper 1,083 C., nickel 1,453 C.) and well below the melting temperature of niobium and of that of the alloy formed by the vector metal and the refractory metals. It is this reduction of the operating temperature on the one hand and the uniformity of the product obtained on the other which form the whole advantage of the method, permitting the preparation of this alloy under conditions which can be obtained in practice and on a large scale. Experience has shown that it is possible to work at temperatures lower than 2,000 C., and in particular that at 1,900 C., excellent results are obtained.

The duration of the treatment is governed by the rise in temperature and by obtaining thermo-dynamic equilibrium between the liquid and solid phases. As the temperature increases, this equilibrium is reached more rapidly, and it may be stated by way of example that the duration of the operation at a temperature of l,900 C. is of the order of one to two hours.

The separation of the liquid and solid phases may be carried out by following either of the two methods of operation below:

either by centrifuging when it is necessary to overcome interfacial tensions in order to facilitate separation. In this case, the separation takes place at the end of the preparation of the solid solution, in the same experimental apparatus;

or by decantation of the phases in which the differences of density are sufficient;

or by differential electromagnetic separation of the phases. In this case, the solid phase is retained by the electro-magnetic forces, either on the walls of the crucible or, as the case may be, on a support which can be the sheath of the thermo-couple just above the liquid phase with which it remains in equilibrium through the intermediary of a liquid film.

In order to retain at ambient temperature the composition obtained during equilibrium at the preparation temperature, and when once the phases have been separated, it is necessary to carry out a very rapid solidification of the liquid phase by a tempering operation which is advantageously effected by the circulation of a neutral gas obtained from a cold source. In this way, there is prevented any exchange at the level of the liquid-solid interfaces, and there is thus obtained an alloy which has the greatest possible homogeneity. After this, the solid solution with a base of niobium and tungsten and/or molybdenum can be collected by a simple mechanical operation, while the alloy with a base of carrier metal is reutilized.

The solid solution thus collected has the appearance of a mass of grains of homogeneous composition, all having the same concentration of niobium and tungsten or molybdenum, coated with a fine film of alloy produced by the liquid phase.

Micrographic examination of the product shows that the grains are constituted by a single homogeneous phase and not by a composite agglomerate of niobium with tungsten and/or molybdenum. The main characteristics of this alloy depend on the respective quantities of niobium with respect to tungsten and/or molybdenum, but they show inter alia excellent refractory qualities.

The characteristics and advantages of the invention will be further brought out in the description which follows below of examples of its utilization, reference being made to the accompanying drawings, in which:

FIG. 1 is a view in cross-section of a crucible for separating by centrifuging, the solid and liquid phases being shown at the end of the centrifuging operation;

FIG. 2 is a view in cross-section of a crucible for separating by electro-magnetic process, the liquid and solid phases being shown at the end of the separation operation.

EXAMPLE 1 There are available copper (Cu) serving as the carrier metal, niobium (Nb) and tungsten (W). The vector metal can be utilized in very diverse forms; ingots, grains, etc., whereas it is advantageous for the refractory metals to be in the form of coarse powder, which permits the process of formation of the solid solution to be accelerated. It should be noted in passing that the choice of the crucible depends essentially on the working temperature: at 1,600 C., an alumina crucible may be quite adequate, between 1,-600 C. and 2,000 C., a zirconia crucible is chosen, and above 2,000 C., it is necessary to employ a crucible of beryllium oxide.

A mixture is made of 85 grams of Cu, 10 grams of Nb, and grams of W, which corresponds to the following proportions given in percentages of the total weight:

This mixture is introduced into a cylindrical crucible of the type shown in FIG. 1, comprising an outer body 2 of graphite and an internal lining 3 of zirconia. A thermometer probe 4 is engaged in the crucible and heating is effected by electro-magnetic induction. Means (not shown) are provided for maintaining a neutral atmosphere of argon in and around the crucible, these means further incorporating means for rapid putting into circulation of cold gas.

The method of operation is as follows:

The crucible is placed in a furnace which is previously exhausted to a vacuum of torr and then filled with inert gas (argon or helium) at atmospheric pressure. Heating is carried out by induction either at medium frequency of the order of 10 kc. or at high frequency of the order of 400 kc. The rise in temperature, programmed following a linear function of the power supply to the furnace, permits the desired temperature to be obtained in a fairly short time (for example 30 minutes to reach l,900 C.). The mixture is heated to l,900 C. and is held at this tmeperature for 1 hour 30 minutes. At the end of this operation, the crucible is set in rotation at 2,000 r.p.m. for 15 minutes, while maintaining the temperature at 1,900 C. Then, while continuing the centrifuging movement, abrupt cooling is effected, either naturally by simply stopping the heating (rate of cooling of the order of 950 C./min.) or by tempering the alloy by a rapid circulation of argon or helium round the crucible.

There are obtained in the crucible two quite distinct compounds, as shown in FIG. 1. On the one hand, at the bottom of the crucible, a mass S with an upper contour C defined by a complex surface of blistered form, and on the other hand, an annular mass L, the internal contour C of which is a cylindrical surface.

The mass S having a weight of 8 grams is constituted solely by niobium and tungsten, the proportions being 28% Nb and 72% W. It has been verified that this originates from a solid solution of these two substances.

On the other hand, the annular mass L has the form of a metal structure comprising an alloy of 7% of niobium and 93% of copper.

EXAMPLE 2 The procedure is the same as that indicated in Example l, but using a mixture of 60 grams of Cu, 32 grams of Nb and 8 grams of W, corresponding to the following proportions:

The centrifuging time is limited to 5 minutes and there are obtained:

A solid solution of 12 grams composed of 50% Nb and A liquid of 88 grams composed of 67% of a Nb-Cu alloy and 33% of Cu.

EXAMPLE 3 There was employed a crucible with electromagnetic separation (FIG. 2) utilizing an induction heating means at a medium frequency of the order of 10 kc.; there is again employed a crucible 11 with an outer body 12 of graphite and an inner lining 13 of zirconia, equipped with a thermometer probe 14. In this case the crucible is not rotatably mounted, but the inductors 15 cause the production in the mixture placed in the crucible, of electric currents which are utilized not only to ensure the heating to 1,900 C., but also for the electro-magnetic separation of the solid phase from the liquid phase.

In this way, there have been prepared grams of a mixture of 18% of Ni, 46% of Nb, 36% of W, and the heating was carried out to 1,900 C. for two hours with r a medium frequency current automatically ensuring the electro-magnetic separation. After tempering and cooling the alloy there was found in the bottom of the crucible a compact mass L' derived from the liquid phase and a solid mass S' in the shape of a heart round the thermometer probe and overhanging the liquid mass. The mass S has a weight of 32 grams and its composition is 25% Nb and 75% W.

EXAMPLE 4 A mixture of 100 grams was prepared, constituted by:

'Cu 70%-Nb 15%-Mo 15% The procedure is the same as in Example 1, with heating to 1,900 C. for 1 hour 30 minutes, followed by centrifuging for 15 minutes at 2,000 r.p.m., while maintaining the temperature at 1,900" C., and by tempering during the terminal phase of the centrifuging operation. There was obtained:

at the bottom of the crucible, a mass of 24 grams originating from the solid solution, forming an alloy Nb (43%) with Mo (57%);

a mass of liquid origin of 76 grams, forming an alloy composed of 91% of Cu, of Nb and 4% of Mo, this mass surmounting the solid mass.

EXAMPLE 5 The same procedure is followed as in Example 3, but with a crucible having no thermometer probe 14, the measurement of temperature being effected by optical pyrometry with electro-magnetic separation, with a mixture of 100 grams of 30% of Cu, 61% of Nb and 9% of M0, which is heated by induction with a medium frequency current for 2 hours at l,900 C. Electra-magnetic separation of the phases resulted in:

77 grams of a mass of liquid origin constituting an alloy of 45% Cu, 50% Nb, 5% of Mo;

33 grams of a mass of solid origin surmounting the liquid mass formed by an alloy of 85% of Nb and 15% of Mo distributed over the walls of the crucible.

In carrying out the method according to the invention, there are established the isotherm sections of the diagrams of carrier metal equilibrium in the presence of the refractories, which makes it possible, following the usual technique in metallurgy, to determine the quantities of the various metals which it is convenient to take at the start in order to obtain the final composition of the solid solution.

The applications of these alloys thus obtained are those which correspond, amongst others, to their refractory capacity and, if so desired, to their use as super-conductors. The alloys thus obtained are reduced to powder by grinding and utilized for the manufacture of any object following the usual techniques of powder metallurgy.

For example, alloys with a base of niobium can be employed for the manufacture of parts of aeronautical devices. It should be noted that the residual presence or traces of the alloy coming from the liquid phase does not offer any disadvantage. On the contrary, in the technique of powder metallurgy, in particular of refractory metals, use is made of these elements approximately in the same quantities, so that their addition in the method according to the invention becomes useless.

What we claim is:

1. A method of preparing an alloy consisting essentially of refractory metals, which method comprises the steps of:

mixing said refractory metals with a non-refractory metal serving as a carrier, at a temperature below the melting point of any of said metals;

said non-refractory metal being highly soluble in one of said refractory metals when in a liquid state, but only slightly soluble in the alloy to be prepared;

heating the resulting mixture in an inert atmosphere to a temperature higher than the melting point of said non-refractory metal but lower than the melting point of the refractory metal having the lowest melting point and also lower than the melting point of an alloy of said refractory and non-refractory metals;

maintaining said temperature until an equilibrium is reached between a solid phase consisting of a homogeneous solid solution consisting essentially of said refractory metals and a liquid solution comprising at least one refractory metal with said non-refractory metal;

separating said solid phase from said liquid phase; and

rapidly cooling the mixture to solidify said liquid phase.

2. A method according to claim 1, wherein the refractory metals used are selected from the group consisting of niobium, tungsten and molybdenum and said temperature is maintained for from one to two hours.

3. A method according to claim 1, wherein the nonrefractory metal used is copper.

4. A method according to claim 1, wherein the nonrefractory metal used is nickel.

5. A method as claimed in claim 1 for preparing an alloy of refractory metals selected from the group consisting of niobium, tungsten and molybdenum, wherein said solid phase is separated from said liquid phase-by centrifugation.

6. A method as claimed in claim 1 of preparing an alloy of refractory metals selected from the group consisting of niobium, tungsten and molybdenum, wherein said solid phase is separated from said liquid phase by electro-magnetic separation.

7. A method according to claim 6, wherein said mixture is heated by electro-magnetic induction means, which also constitute the electro-magnetic separating means.

8. A method according to claim 7, wherein the alloy is formed at a temperature of the order of l,900 C.

References Cited UNITED STATES PATENTS 2,030,229 2/1936 Schwarzkopf 135 2,452,665 11/1948 K1'01l et al. 7563 3,019,102 1/ 1962 Saarivirta 75135 3,084,041 4/1963 Zegler 75135 WINSTON A. DOUGLAS, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 7584, 

